Lattices or polymer particles obtained therefrom are widely used as coatings, adhesives, ink and painting materials, for precision mold constructions and the manufacture of micro-sized materials. The unique properties of micro- and nanoscaled polymer particles with specific properties such as defined molecular weight distributions and polydispersities have meanwhile gained further significant attention not only in the electronics industry, for example, in the manufacture of TFT and LCD displays, digital toners and e-paper, but also in the medical sector, such as for drug delivery systems, diagnostic sensors, contrast agents and many other fields of industry.
Polymer nanoparticles are frequently synthesized by physical methods such as evaporation of polymer solution droplets or, in particular, for commercially important polymers such as polystyrene and poly(meth)acrylates, by direct synthesis of nanoparticles using special polymerisation processes. The most common processes are heterophase polymerisations, in particular, thermally or photo-initiated emulsion polymerizations.
A vast number of radical polymerization processes were developed over the last decades to achieve a better control over the polymerisation process and thereby the properties of the resulting polymer nanoparticles.
Emulsion polymerisations induced by X-ray radiation are described in S. Wang, X. Wang, Z. Zhang, Eur. Polym. J., 2007, 43, 178.
Emulsion polymerisations induced by UV/Vis radiation are described in P. Kuo, N. Turro, Macromolecules 1987, 20, 1216-1221, wherein the formation of polystyrene nanoparticles having a weight average molecular weight of 500 kg/mol or less is disclosed.
In T. Ott, Dissertation ETH Zürich No. 18055, 2008, Chapter 6, batch emulsion polymerisations induced by photofragmentation of bisacylphosphines are described in detail. However, high monomer conversion typically requires irradiation times of more than 2 hours.
A. Chemtob et al. describes a batch process (in a cuvette) for the preparation of lattices comprising copolymers by irradiating a miniemulsion of nanodroplets comprising acrylic acid, butylacrylate and methylmethacrylate encapsulating high amounts (4 wt.-%) of a hydrophobic photoinitiator of the BAPO type (BAPO=bisacylphosphine oxide).
The aforementioned processes are not industrially applicable or commercially attractive, however, due to their long reaction times and/or limited throughputs.